1. Field of the Invention
The present invention relates to a device for detecting the misfire in an internal combustion engine and, particularly, to a device for detecting the misfire in an internal combustion engine featuring improved reliability in all operation regions by detecting the misfire using either the revolution data or the ionic current depending upon the operation region.
2. Prior Art
A device for detecting the misfire in an internal combustion engine relying on a change in the pulse period ratio: as revolution data, has heretofore been well known as disclosed in, for example, Japanese Unexamined Patent Publication (Kowai) No. 79396/1993 (Japanese Patent No. 2606019).
There has also been known a device for detecting the misfire in an internal combustion engine relying on the detection levee of an ionic current as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 26090/1993 (Japanese Patent No. 2657012).
However, no device for detecting the misfire based on both the revolution data and the ionic current has yet been proposed.
It has generally been known that the reliability (S/N ratio) for detecting the misfire based on the revolution data and the ionic current, varies depending upon the operation region (rotational speed) of the internal combustion engine.
FIGS. 6 and 7 are diagrams of characteristics showing the reliability (S/N ratio) for detecting the period ratio and the ionic current relative to the rotational speed Ne [rpm] of the engine.
Referring to FIG. 6, the reliability for judging the misfire based on the period ratio (revolution data) deteriorates with an increase in the rotational speed Ne of the engine.
This is because, the rotational pulse period and the amount of change thereof decrease with an increase in the rotational speed Ne of the engine, and it becomes difficult to detect a change in the period ratio.
Here, it is not desirable to operate a period ratio within a short pulse section in a high-speed operation region, since the load to the computer (ECU) increases.
Referring to FIG. 7, furthermore, the reliability for judging the misfire based on the ionic current deteriorates with a decrease in the rotational speed Ne of the engine, and particularly deteriorates near the idling rotational speed (700 rpm) as indicated by a broken line.
This is because, in the idling operation region, the level for detecting the ionic current loses stability since the load exerted on the engine and the rotational speed of the engine vary to a great extent. Furthermore, the level for detecting the ionic current temporarily drops depending upon the fouling of the spark plugs and the properties of the fuel.
According to the conventional devices for detecting the misfire in the internal combustion engines as described above, the reliability is deteriorated during the high-speed operation when the period ratio (revolution data) is relied upon and the reliability is deteriorated during the low-speed operation when the ionic current is relied upon.